Referring to FIG. 1, conventional light emitting diodes or “LEDs” include thin layers of semiconductor material of two opposite conductivity types, typically referred to as p-type layers 20 and n-type layers 22. The layers 20, 22 are typically disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at an opposite end of the stack. Each LED includes a junction 24 provided at the interface of the p-type and n-type layers. The various layers of the stack may be deposited in sequence on a substantially transparent substrate 26, such as a sapphire substrate, to form a wafer. The wafer is then cut apart to form individual dies which constitute separate LEDs.
In operation, electric current passing through the LED package is carried principally by electrons in the n-type layer 22 and by electron vacancies or “holes” in the p-type layer 24. The electrons and holes move in opposite directions toward the junction 24, and recombine with one another at the junction. Energy released by electron-hole recombination is emitted from the LED as light 28. As used herein, the term “light” includes visible light rays, as well as light rays in the infrared and ultraviolet wavelength ranges. The wavelength of the emitted light 28 depends on many factors, including the composition of the semiconductor materials and the structure of the junction 24.
FIG. 2 shows a typical LED package 10 including p-type and n-type semiconductor layers 20, 22 mounted atop a substantially transparent substrate 26. The LED is surrounded by a substantially transparent encapsulant 30. Each layer of the package has its own unique index of refraction. As used herein, the term “refraction” means the optical phenomenon whereby light entering a transparent medium has its direction of travel altered. The LED 18 has an index of refraction designated n1, the transparent substrate 26 has an index of refraction designated n2 and the encapsulant layer 30 has an index of refraction designated n3. Because the index of refraction n2 of the substantially transparent substrate 26 is greater than the index of refraction n3 of the transparent encapsulant 30, many of the light rays generated by LED 18 will not be emitted from the LED package 10, but will be subject to total internal reflection. The optical phenomenon, known as total internal reflection, causes light incident upon a medium having a lesser index of refraction (e.g. encapsulant layer) to bend away from the normal so that the exit angle is greater than the incident angle. The exit angle will then approach 90° for some critical incident angle θc, and for incident angles θi greater than critical angle θc there will be total internal reflection of the light ray. The critical angle can be calculated using Snell's Law. Referring to FIG. 2, a light ray subject total internal reflection is designated as light ray 32.
Thus, in many LED packages the light rays generated by the LED are never emitted from the package because such light rays are totally internally reflected within the package. Thus, there is a need for LED packages having designs that optimize the amount of light that may be extracted from the packages. There is also a need for LED packages having means for tailoring the emission geometry for higher efficiency in advanced packages.